Studies on the mechanism of Fv-1 dependent cell-mediated resistance to murine leukemia viruses: Studies are in progress to determine the biochemical and biological basis for the restriction of MSV pseudotypes in nonpermissive cells as well as for the ability of tropic MSV to overcome the restriction. The pathological studies of BALB/C mice infected with comparable titers of several cloned MSV stocks will be completed and analyzed for significant differences among the MSV isolates. Studies on S plus L minus cells superinfected with murine leukemia viruses: The biology of defective MuLV clones will be studied and correlated with any immunological or biochemical deficiencies in the virions themselves.